@Khorvog_Karbossa we were told by a dev that the formula used is:
% chance to resist = tenacity - potency with a minimum chance of 15%
So a toon with 100% tenacity would have a 50% chance to resist an effect applied by an enemy with 50% potency. So the general consensus is that tenacity is pretty pointless.
Potency has no effect on applying positive effects. They simply have the chance stated (and if there is no chance stated then they automatically work unless there is something else like shock on the toon).